Plants forming dense clones. Roots arising at nodes. Petioles 3.1--5.7 cm, sparsely pubescent. Pinnae 6--15 × 5--14 mm, pilose. Sporocarp stalks erect, unbranched, attached at base of petiole, sometimes bent near apex, 5--10 mm. Sporocarps nodding, 5--6 × 3.6--4 mm, 1.5 mm thick, ovate in lateral view, covered with long straight hairs when young but eventually glabrate; raphe 0.8--1 mm, proximal tooth 0.2--0.6 mm, curved away from sporocarp, distal tooth up to 0.4 mm, broad, blunt or absent. Sori 14--20.

Sporocarps produced summer--fall (Jun--Oct). Around ponds and marshes, in wet depressions in sagebrush and less commonly on river margins; 700--2400 m; Calif., Idaho, Mont., Nev., Oreg., Utah, Wash., Wyo.

Marsilea oligospora recently has been resegregated from M . vestita (D. M. Johnson 1986), from which it differs consistently in its nodding sporocarps that lack a pronounced distal tooth and its pilose leaves and stems. Where their ranges overlap, M . oligospora also has longer sporocarp stalks than does M . vestita . Plants of this species were recently grown from spores 100 years old (D. M. Johnson 1985).